Conventionally, a method for measuring a surface profile of a target plane has been performed as follows. This method involves: emitting a monochromatic light-beam outputted from a monochromatic light source to a reference plane arranged in a posture so as to be obliquely tilted at an optional angle relative to a traveling direction of the light-beam divided by a dividing means and a target plane; and taking one image of an interference fringe generated by the reflected light-beams which are reflected from both the target plane and the reference plane and, then, return on a single optical path. From image data acquired by this image taking operation, first, intensity value data of the interference fringe is obtained on a pixel basis. That is, there is used the following equation which is a computational algorithm for obtaining intensity value data g(x).g(x)=a(x)+b(x)cos {2πfx+φ(x)}
Herein, a(x) represents a DC component contained in an interference fringe waveform having the intensity value data g(x) of the light-beam, b(x) represents an AC amplitude contained in the interference fringe waveform, f represents a spatial frequency component of the intensity value data g(x) of the light-beam, and φ(x) represents a phase corresponding to a predetermined pixel on the target plane.
Examples of a method for obtaining the phase φ(x) include a Fourier transform method, a spatial-phase synchronization method and the like, as described in Patent Document 1.
That is, when a phase is obtained for each pixel, the phase of each pixel is substituted into an equation, z(x)=[φ(x)/4π]λ+z0, which is a computational algorithm for calculating, for each pixel, a surface height of a target plane. Then, a surface profile is specified based on data of these surface heights z(x). Herein, z0 represents a reference height of a single sample surface.
Non-patent Document 1: “Precision Engineering”, Vol. 64, No. 9, pp. 1290-1291 (1998)